Make Sampler completely Fitter-agnostic

[rozyczko]

Addressing @damskii9992's issue in #287 (comment)

Made Sampler fully Fitter-independent.

This is a separate PR on top of #287 to allow for easier review.

[codecov]

Codecov Report

❌ Patch coverage is 98.18182% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.82%. Comparing base (82a1b01) to head (5f8bd2f).

Files with missing lines	Patch %	Lines
src/easyscience/fitting/reshaping.py	96.61%	2 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           sampler-engine-structure-280     #310      +/-   ##
================================================================
+ Coverage                         83.64%   83.82%   +0.17%     
================================================================
  Files                                68       69       +1     
  Lines                              5266     5285      +19     
================================================================
+ Hits                               4405     4430      +25     
+ Misses                              861      855       -6     

Flag	Coverage Δ
unittests	83.82% <98.18%> (+0.17%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/easyscience/fitting/fitter.py	96.03% <100.00%> (+4.70%)	⬆️
src/easyscience/fitting/multi_fitter.py	100.00% <100.00%> (+1.38%)	⬆️
src/easyscience/fitting/sampler.py	100.00% <100.00%> (ø)
src/easyscience/fitting/reshaping.py	96.61% <96.61%> (ø)